Composition for dip forming, dip-formed object, and process for producing the same

ABSTRACT

A dip-formed article which is shaped from a dip-formable composition comprising 100 weight parts of a conjugated diene copolymer prepared by copolymerization of 50 to 85 weight % of a conjugated diene monomer with 15 to 40 weight % of an ethylenically unsaturated nitrile monomer, 1 to 4 weight % of an ethylenically unsaturated acid monomer and 0 to 30 weight % of a copolymerizable other ethylenically unsaturated monomer, 2 to 6 weight parts of sulfur and 0 to 0.2 weight parts of zinc oxide. The dip-formed article is characterized as a glove by being well elongated and ease in donning, being not easily broken upon donning, and not wearying a wearer&#39;s hand even when it is worn for a long time, and having improved lastingness of the tight conformity to the skin of a wearer&#39;s hand.

TECHNICAL FIELD

[0001] This invention relates to a dip-formable composition, adip-formed article and a method for making the dip-formed article. Moreparticularly, it relates to a dip-formed article such as a glove whichis well stretched, can be easily donned, is not easily broken, and, evenwhen it is worn for a long time, it does not weary a wearer's hand andits tight conformity to the skin of a wearer's hand lasts for a longtime; a method for making the dip-formed article; and a dip-formablecomposition capable of giving the dip-formed article.

BACKGROUND ART

[0002] Rubber articles such as a glove, a finger cot and a dropper areproduced by a method generally called “dip-forming”. Of these rubberarticles, gloves are widely used in a household field and an industrialfield. For these uses, it is required that the rubber articles are notbroken even when they are used for a long period of time, and they havehigh resistance to solvents.

[0003] In recent years, the use of a glove is extended to a medicalfield including a surgical operation, and an electronic part-productionfield. In these uses, a glove of a relatively small size is worn in anexpanded state to enhance its tight conformity to the skin of a wearer'shand. Therefore, the following characteristics are required for a glove:(i) ease in donning, i.e., capability of being well stretched and easilydonned, (ii) good followability, i.e., capability of being easilystretched and shrunk so as to follow the movement of wearer's fingers sothat, even when it is worn for a long time, it does not weary a wearer'shand, (iii) good and lasting tight conformity, namely, when it isdistorted by the movement of wearer's fingers, it is loosened or creasedwith wrinkles only to a slight extent, and tight conformity to the skinof a wearer's hand lasts for a long time.

[0004] If a glove for surgical operation has pinholes or is brokenduring operation, infections tend to be caused by bacteria or virus. Ifa glove for electronic part-production has pinholes or is broken duringoperation, insulation failure is liable to occur due to electrolytescontained in sweat and grease from the human body with the result ofmalfunction of electronic parts. Therefore, it is required that a glovehas no pin holes and is not easily broken during operation.

[0005] Natural rubber latex has heretofore been used for gloves for theabove-mentioned uses. However, gloves made from natural rubber latexhave a problem such that wearers acquire allergy thereto. An attempt ismade for the removal of allergic substances, but the steps required arecomplicated and the production cost is increased.

[0006] As a substitute for natural rubber latex, a soft glove havinghigh tensile strength and chemical resistance, which is comprised of anitrile rubber composition containing no zinc oxide, has been proposedin international publication WO 97/48,765. However, examination of thenitrite rubber glove by the present inventors has revealed that thefollowability and lastingness of the tight conformity to the skin of awearer's hand are insufficient.

[0007] A glove having characteristics such that modulus in tension at100% elongation becomes substantially zero in 6 minutes from the startof elongation, which is comprised of a nitrile-butadiene rubber and aminor amount of zinc oxide, has been proposed in U.S. Pat. No.5,014,362. However, this glove has poor lastingness of tight conformity.

DISCLOSURE OF THE INVENTION

[0008] In view of the foregoing, a primary object of the presentinvention is to provide a dip-formed article such as a glove which isnot easily broken and can be easily donned, and has enhanced lastingnessof tight conformity to the skin of a wearer's hand; a method for makingthe dip-formed article; and a dip-formable composition giving thedip-formed article.

[0009] To attain the above-mentioned object, the present inventorsconducted researches and have found that, when a conjugated dienecopolymer prepared by copolymerization of a conjugated diene monomerwith an ethylenically unsaturated nitrile monomer and an ethylenicallyunsaturated acid monomer is vulcanized with a relatively large amount ofsulfur, the resulting vulcanizate exhibits remarkable characteristicswhich have not been seen in the hitherto proposed rubber vulcanizates.Based on this finding, the present invention has been completed.

[0010] Thus, in one aspect of the present invention, there is provided adip-formed article which is shaped from a dip-formable compositioncomprising a conjugated diene copolymer prepared by copolymerization ofa conjugated diene monomer with an ethylenically unsaturated nitrilemonomer and an ethylenically unsaturated acid monomer, and sulfur; andwhich has a modulus in tension at 300% elongation of not larger than 3MPa, a tensile strength at break of at least 10 MPa, an elongation atbreak of at least 500% and a modulus retention of at least 50%.

[0011] In another aspect of the present invention, there is provided adip-formable composition comprising 100 parts by weight of a conjugateddiene copolymer comprising 50% to 85% by weight of conjugated dienemonomer units, 15% to 40% by weight of ethylenically unsaturated nitrilemonomer units, 1% to 4% by weight of ethylenically unsaturated acidmonomer units and 0% to 30% by weight of copolymerizable otherethylenically unsaturated monomer units, 2 to 6 parts by weight ofsulfur and 0 to 0.2 part by weight of zinc oxide.

[0012] In still another aspect of the present invention, there isprovided a method for making a dip-formed article comprising dip-formingthe above-mentioned dip-formable composition.

BEST MODE FOR CARRYING OUT THE INVENTION

[0013] The invention will now be described in detail.

[0014] The conjugated diene copolymer used for the dip-formed article ofthe present invention comprises conjugated diene monomer units,ethylenically unsaturated nitrile monomer units, ethylenicallyunsaturated acid monomer units, and optional units derived from otherethylenically unsaturated monomers copolymerizable with these monomers.

[0015] The conjugated diene monomer is not particularly limited, and asspecific examples thereof, there can be mentioned 1,3-butadiene,isoprene, 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene,1,3-pentadiene and chloroprene. Of these, 1,3-butadiene is preferable.These conjugated diene monomers may be used either alone or as acombination of at least two thereof. The amount of the conjugated dienemonomer units is preferably in the range of 50% to 85% by weight, morepreferably 60% to 81% by weight and especially preferably 65% to 76% byweight, based on the weight of the total monomer units. If the amount ofconjugated monomer units is too small, a glove has poor conformity tothe skin of a wearer's hand. In contrast, if the amount of conjugateddiene monomer units is too large, a glove tends to be easily broken upondonning.

[0016] No limitation is imposed to the ethylenically unsaturated nitrilemonomer, and as specific examples thereof, there can be mentionedacrylonitrile, methacrylonitrile, fumaronitrile andα-chloroacrylonitrile. These ethylenically unsaturated nitrile monomersmay be used either alone or as a combination of at least two thereof.The amount of the ethylenically unsaturated nitrile monomer units ispreferably in the range of 15% to 40% by weight, more preferably 17% to35% by weight and especially preferably 22% to 32% by weight, based onthe weight of the total monomer units. If the amount of ethylenicallyunsaturated nitrite monomer units is too small, a glove tends to beeasily broken upon donning and its tight conformity to the skin of awearer's hand does not last for a long period. In contrast, if theamount of ethylenically unsaturated nitrile monomer units is too larger,a glove has poor conformity to the skin of a wearer's hand.

[0017] No limitation is imposed to the ethylenically unsaturated acidmonomer provided that the monomer has an acid group such as, forexample, a carboxyl group, a sulfonic acid group or an acid anhydridegroup. As specific examples of the ethylenically unsaturated acidmonomer, there can be mentioned ethylenically unsaturated carboxylicacid monomers such as acrylic acid, methacrylic acid, itaconic acid,maleic acid and tumaric acid; polycarboxylic anhydride monomers such asmaleic anhydride and citraconic anhydride; ethylenically unsaturatedsulfonic acid monomers such as styrenesulfonic acid; and ethylenicallyunsaturated polycarboxylic acid partial ester monomers such as monobutylfumarate, monobutyl maleate and 2-hydroxypropyl maleate. Theseethylenically unsaturated acid monomers may be used in the form of analkali metal salt or an ammonium salt. Of these, ethylenicallyunsaturated carboxylic acid monomers are preferable. Methacrylic acid isespecially preferable. These ethylenically unsaturated acid monomers maybe used either alone or as a combination of at least two thereof. Theamount of the ethylenically unsaturated acid monomer units is preferablyin the range of 1% to 4% by weight, more preferably 2% to 3.5% by weightand especially preferably 2% to 3% by weight, based on the weight of thetotal monomer units. If the amount of ethylenically unsaturated acidmonomer units is too small, a glove tends to be easily broken upondonning. In contrast, if the amount of ethylenically unsaturated acidmonomer units is too large, a glove has poor conformity to the skin of awearer's hand and its tight conformity to the skin of a wearer's handdoes not last for a long period.

[0018] Other copolymerizable ethylenically unsaturated monomersoptionally used for copolymerization include, for example, aromaticvinyl monomers such as styrene and alkylstyrenes; ethylenicallyunsaturated amide monomers such as acrylamide, methacrylamide,N-methylolacrylamide and N-methylolmethacrylamide; ethylenicallyunsaturated carboxylic acid ester monomers such as methyl acrylate,methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate,butyl methacrylate, 2-ethylhexyl acrylate and 2-ethylhexyl methacrylate;and crosslinking monomers such as divinylbenzene, diethylene glycoldiacrylate, diethylane glycol dimethacrylate, pentaerythritol acrylateand pentaerythritol mathacrylate. The amount of the copolymerizableother ethylenically unsaturated monomer units is preferably in the rangeof 0% to 30% by weight, more preferably 0% to 21% by weight andespecially preferably 0% to 11% by weight, based on the weight of thetotal monomer units.

[0019] As a preferable example of the conjugated diene copolymer used inthe present invention, there can be mentioned conjugateddiene-ethylenically unsaturated nitrile-ethylenically unsaturated acidcopolymers. Of these, a butadiene-acrylonitrile-methacrylic acidcopolymer is especially preferable. This copolymer comprises, based onthe weight of copolymer, preferably 50% to 85% by weight, morepreferably 61.5% to 81% by weight and especially preferably 65% to 76%by weight of butadiene units, preferably 15% to 40% by weight, morepreferably 17% to 35% by weight and especially preferably 22% to 32% byweight of acrylonitrile units, and 1% to 4% by weight, more preferably2% to 3.5% by weight and especially preferably 2% to 3% by weight ofmethacrylic acid units.

[0020] The conjugated diene copolymer preferably contains not largerthan 50% by weight, more preferably not larger than 25% by weight andespecially preferably not larger than 10% by weight of ingredientsinsoluble in methyl ethyl ketone. When the content of methyl ethylketone-insoluble ingredients in the copolymer is within this specifiedrange, a rubber glove having well-balanced characteristics can beobtained.

[0021] The fractions of the conjugated diene copolymer, which aresoluble in tetrahydrofuran, preferably have a weight average molecularweight in the range of 50,000 to 500,000, more preferably 80,000 to200,000 as expressed in terms of that of polystyrene (said weightaverage molecular weight is hereinafter referred to merely as “molecularweight”). When the molecular weight of copolymer is within thisspecified range, a rubber glove having well-balanced characteristics canbe obtained.

[0022] The procedure by which the conjugated diene copolymer is producedis not particularly limited, and, for example, an emulsionpolymerization procedure and a solution polymerization procedure can beadopted. Among these an emulsion polymerization procedure is preferredbecause an as-obtained copolymer latex can be used without a specialtreatment as a main ingredient of the dip-formable composition.

[0023] As specific examples of emulsifiers used for emulsionpolymerization, there can be mentioned nonionic emulsifiers such aspolyoxyethylene alkyl ethers, polyoxyethylene alkylphanol ethers,polyoxyethylene alkyl asters and polyoxyethylene sorbitan alkyl asters;anionic emulsifiers such as fatty acid salts, for example, salts ofmyristic acid, palmitic acid, oleic acid and linolenic acid,alkylbenzenesulfonic acid salts such as sodium dodecylbenzenesulfonate,higher alcohol sulfuric acid ester salts, and alkylsulfosuccinic acidsalts; cationic emulsifiers such as alkyltrimethylammonium chlorides,dialkylammonium chlorides and benzylammonium chloride; andcopolymerizable emulsifiers such as sulfoesters of α, β-unsaturatedcarboxylic acids, sulfate esters of α, β-unsaturated carboxylic acidsand sulfoalkyl aryl ethers. Of these, anionic emulsifiers are preferablyused. These emulsifiers may be used either alone or as a combination ofat least two thereof. The amount of emulsifiers is in the range of about1 to 10 parts by weight, more preferably 2 to 6 parts by weight, basedon 100 parts by weight of the monomer mixtures.

[0024] As specific examples of polymerization initiators, there can bementioned inorganic peroxides such as sodium persulfate, potassiumpersulfate, ammonium persuldate and hydrogen peroxide; organic peroxidessuch as diisopropylbenzene hydroperoxide, cumene hydroperoxide, t-butylhydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide,2,5-dimethylhexane-2,5-dihydroperoxide, di-t-butyl peroxide, di-α-cumylperoxide, acetyl peroxide, isobutyryl peroxide and benzoyl peroxide; andazo compounds such as azobisisobutyronitrile,azobis-2,4-dimethylvaleronitrile and methyl azobisisobutyrate. Of these,inorganic peroxides are preferable because a copolymer latex can bestably produced. These polymerization initiators may be used eitheralone or as a mixture of at least two thereof. The amount ofpolymerization initiators varies depending upon the particular kindthereof but is in the range of about 0.1 to 1 part by weight based on100 parts by weight of the monomer mixture.

[0025] The peroxide can be used in combination with a reducing agent,namely, as a redox polymerization initiator. The reducing agent usedincludes, for example, compounds containing a metal ion of a reducedstate such as ferrous sulfate cupreous naphthenate; sulfonic acidcompounds such as methanesulfonic acid; and amine compounds such asdimethylaniline. These reducing agents may be used either alone or as acombination of at least two thereof. The amount of reducing agentsvaries depending upon the particular kind thereof but is in the range of0.03 to 10 parts by weight based on 1 part by weight of a peroxide.

[0026] Among the above-recited polymerization initiators, a redoxpolymerization initiator comprising a combination of a peroxidepolymerization initiator with a reducing agent is preferable.

[0027] In emulsion polymerization, a molecular weight modifier ispreferably used to control the molecular weight of copolymer. Asspecific examples of the molecular weight modifier, there can bementioned mercaptans such as n-butylmercaptan and t-dodecylmercaptan;sulfides such as tatraethylthiuram sulfide and dipentamethylenethiuramhexasulfide; and an α-methylstyrene dimer and carbon tetrachloride. Ofthese, mercaptans are preferable. t-dodecylmercaptan is especiallypreferable. These molecular weight modifiers may be used either alone oras a mixture of at least two thereof. The amount of molecular weightmodifier may be chosen so that the molecular weight of copolymer fallswithin a desired range, but its amount is in the range of about 0.1 to 2parts by weight, preferably 0.3 to 1 part by weight, based on 100 partsby weight of the monomer mixture.

[0028] The amount of water used for emulsion polymerization is in therange of about 80 to 600 parts by weight, preferably 100 to 200 parts byweight, based on 100 parts by weight of the monomer mixture.

[0029] If desired, polymerization auxiliaries such as a particle sizecontroller, an age resister, a chelating agent and an oxygen catcher canbe used.

[0030] The manner in which a monomer mixture is added is notparticularly limited, and includes, for example, a manner in which amonomer mixture is charged in one lot in a polymerization vessel, amanner in which a monomer mixture is continuously fed into apolymerization vessel, and a manner in which a part of monomer mixtureis initially charged in a polymerization vessel and the remainder iscontinuously added therein.

[0031] The composition of the monomer mixture is chosen in considerationof the polymerization conversion and other factors so that a copolymerhaving a desired composition is obtained.

[0032] In emulsion polymerization, the polymerization temperature ispreferably in the range of 10 to 60° C., more preferably 30 to 45° C.,and the polymerization time is in the range of about 5 to 30 hours. Thepolymerization conversion is preferably at least 90% by weight, morepreferably at least 92% by weight.

[0033] The copolymer latex as obtained by emulsion polymerization has aparticle diameter preferably in the range of 60 to 300 nm, morepreferably 80 to 150 nm, as number average particle diameter as measuredby a transmission electron microscope. The particle diameter can bedesirably controlled by varying the kind of emulsifier or the amount ofpolymerization initiator or by other means.

[0034] After the emulsion polymerization proceeds until thepolymerization conversion reaches a desired value, if desired, apolymerization stopper is added to terminate the reaction, and further,if desired, unreacted monomers are removed, and then, the solidconcentration and pH value of the as-obtained copolymer latex areadjusted to predetermined values to give a desired copolymer latex.

[0035] The adjustment of pH value of a copolymer latex is conducted byaddition of a basic substance. The basic substance used includes, forexample, alkali metal hydroxides such as lithium hydroxide, sodiumhydroxide and potassium hydroxide; alkali metal carbonates such assodium carbonate and potassium carbonate; alkali metalhydrogencarbonates such as sodium hydrogencarbonate; ammonia; andorganic amine compounds such as trimethylammonium and triethanolamine.Of these, alkali metal hydroxides and ammonia are preferably used. Thesebasic substances are added as a solution having a concentration of 1% to40% by weight, preferably 2% to 15% by weight, and preferably as anaqueous solution to avoid occurrence of agglomerates upon addition.

[0036] The dip-formable composition of the present invention comprises100 parts by weight of the above-mentioned conjugated diene copolymer,sulfur and optional zinc oxide.

[0037] The amount of sulfur is preferably in the range of 2 to 6 partsby weight, more preferably 2 to 5 parts by weight, based on 100 parts byweight of the conjugated diene copolymer. If the amount of sulfur is toosmall, a glove tends to be broken upon donning and its tight conformityto the skin of a wearer's hand does not last for a long period. Incontrast, if the amount of sulfur is too large, a glove has poorconformity to the skin of a wearer's hand.

[0038] Zinc oxide is preferably not used. However, when zinc oxide isused, its amount is preferably up to 0.2 part by weight, more preferablyup to 0.1 part by weight, based on 100 parts by weight of the conjugateddiene copolymer. If the amount of zinc oxide is too large, a glove haspoor conformity to the skin of a wearer's hand.

[0039] The dip-formable composition of the present invention preferablyfurther comprises a vulcanization accelerator. As specific examples ofthe vulcanization accelerator, there can be mentioned aldehyde-aminecondensates such as n-butyl aldehyde-aniline condensate and butylaldehyde-monobutylamine condensate; thiurams such as2-mercaptobenzothiazole zinc, tetramethylthiuram monosulfide andtetramethylthiuram disulfide; and dithiocarbamates such as zincdiethyldithiocarbamate and zinc dibutyldithiocarbamate. Of these,thiurams and dithiocarbamates are preferable. These vulcanizationaccelerators are used preferably in combination. The amount ofvulcanization accelerators is preferably in the range of 0.5 to 4 partsby weight, more preferably 1 to 3 parts by weight, based on 100 parts byweight of the conjugated diene copolymer. When the amount ofvulcanization accelerators is within this range, a glove is not easilybroken upon donning and is excellent in ease in donning.

[0040] If desired, the dip-formable composition of the present inventionfurther comprises auxiliaries such as a pH adjuster, an age resister, athickener and a filler. Provided that the object of the presentinvention can be achieved, other latexes such as natural rubber latexand isoprene rubber latex can be added in the composition.

[0041] The method for preparing the dip-formable composition is notparticularly limited. For example, there is adopted a method whereinsulfur, a vulcanization accelerator, a filler, a thickener and otheringredients are incorporated in a conjugated diene copolymer latex byusing a dispersion mixer such as a ball mill, a kneader or a disper.Alternatively, a method can be adopted wherein predetermined ingredientsother than a conjugated diene copolymer are mixed together by using theabove-mentioned dispersion mixer to prepare an aqueous dispersion ofthese ingredients, and then, the copolymer and other remainderingredients are mixed together with the aqueous dispersion.

[0042] The as-prepared dip-formable composition can be subjected toaging. Aging conditions employed vary depending upon the particularcomposition, but usually aging is conducted at a temperature in therange of room temperature to about 40° C. for a period of several hoursto several days.

[0043] The solid concentration in the dip-formable composition ispreferably in the range of 5% to 50% by weight, more preferably 10% to45% by weight and especially preferably 20% to 40% by weight. The pHvalue of the dip-formable composition is preferably in the range of 8 to12 and more preferably 9 to 11. To adjust the pH value of thedip-formable composition, a basic substance can be added which isselected from those recited above for pH adjustment of a conjugateddiene copolymer latex. Among the recited basic substances, alkali metalhydroxides are preferable. Potassium hydroxide is especially preferable.

[0044] The method for making the dip-formed article of the presentinvention is a method for dip-forming the above-mentioned dip-formablecomposition.

[0045] The dip-forming is carried out by a method wherein a form isdipped in a bath of the dip-formable composition whereby thedip-formable composition is deposited over the form, the form is thentaken from the bath, and then the form is dried whereby the compositiondeposited thereon is dried.

[0046] In the dip-forming method, a coagulant can be applied to a formbefore the form is dipped in a bath of the dip-formable composition orafter the form is taken from said bath. More specifically, a form can bedipped in a bath of coagulant whereby the coagulant is deposited on theform before the form is dipped in a bath of the dip-formablecomposition, according to the anode coagulant dip process, or, a formhaving deposited thereon the dip-formable composition can be dipped in abath of coagulant according to the Teague coagulant dip process. Amongothers, the anode coagulant dip process is preferable because adip-formed article having a uniform thickness can be obtained.

[0047] The coagulant includes, for example, water-soluble multivalentmetal salts. As specific examples of the water-soluble multivalent metalsalts, there can be mentioned halides such as barium chloride, calciumchloride, magnesium chloride, zinc chloride and aluminum chloride;nitrates such as barium nitrate, calcium nitrate and zinc nitrate;acetates such as barium acetate, calcium acetate and zinc acetate; andsulfates such as calcium sulfate, magnesium sulfate and aluminumsulfate. Of these, calcium salts are preferable. Calcium nitrate is mostpreferable. These water-soluble multivalent metal salts are preferablyused in an aqueous solution form. The aqueous solution may additionallycontain a water-soluble organic solvent such as methyl alcohol and ethylalcohol. The concentration of a multivalent metal salt in the aqueoussolution varies depending upon the particular kind of multivalent metalsalt, but is preferably in the range of 5% to 70% by weight and morepreferably 20% to 50% by weight.

[0048] In the dip-forming method, a form taken from a bath of thedip-formable composition can be treated with warm water or subjected toheat treatment. By the warm water treatment or the heat treatment,excessive monomers and ingredients are removed and a crosslinkingreaction of the conjugated diene copolymer is accelerated. The processof warm water treatment or heat treatment is not particularly limited,and includes, for example, a process of dipping the form having thedip-formable composition deposited thereon into warm water, a process ofdrying the form having the dip-formable composition deposited thereon bywarm air in an oven, and a process of irradiating the form having thedip-formable composition deposited thereon with infrared rays. Theheating temperature is preferably in the range of 80 to 150° C. and morepreferably 100 to 130° C., and the heating time is in the range of about10 to 120 minutes.

[0049] Before or after the form having the dip-formable compositiondeposited thereon is subjected to warm water treatment or heattreatment, the form is preferably washed with water or warm water toremove water-soluble impurities such as excessive emulsifier andcoagulant.

[0050] The dip-formed article is finally removed from the form. As theremoving process, a process of manually removal or a process of removalby water pressure or compressed air pressure is adopted. Even when thestep of forming the dip-formed article has not been completed, thedip-formed article can be removed from the form in the midway of thestep and thereafter subjected to the remainder treatment.

[0051] In the case where the dip-formed article is a glove, fine organicor inorganic particles such as talc, calcium carbonate or starch can besprayed on the surfaces of glove, or an elastomer layer containing thesefine organic or inorganic particles can be formed on the surfaces ofglove, or, the surfaces of glove can be chlorinated, so that the glovecan be easily donned and put off, or the glove does not stick to eachother when the glove is placed in contact with each other.

[0052] The dip-formed article of the present invention is made from theabove-mentioned dip-formable composition, and has a modulus in tensionat 300% elongation of not larger than 3 MPa, a tensile strength at breakof at least 10 MPa, an elongation at break of at least 500% and amodulus retention of at least 50%.

[0053] The modulus in tension at 300% elongation of the dip-formedarticle is not larger than 3 MPa and preferably not larger than 2.5 MPa.If the 300% modulus is larger than 3 MPa, the followability of glovebecomes poor.

[0054] The tensile strength at break of the dip-formed article is atleast 10 MPa, preferably at least 12 MPa and more preferably at least 14MPa. If the tensile strength is lower than 10 MPa, a glove tends to beeasily broken upon donning.

[0055] The elongation at break of the dip-formed article is at least500% and preferably at least 550%. If the elongation is smaller than500%, a glove is poor in donning.

[0056] The modulus retention of the dip-formed article is at least 50%and preferably at least 60%. If the modulus retention is smaller than50%, the lastingness of tight conformity of glove becomes poor. Themodulus retention is a measure for the difficulty in occurrence ofstress relaxation, and is determined as follows. A specimen punched froma dip-formed article is drawn to an elongation of 100% (namely to alength of twice of the original length), and immediately thereaftermodulus in tension (A) is measured. After the specimen is retained for 6minutes, its modulus in tension (B) is measured. The modulus retentionis expressed by the ratio (B)/(A).

[0057] The dip-formed article of the present invention preferably has anetwork chain concentration in the range of 4×10⁻⁴ to 7×10⁻⁴ mol/cm³,more preferably 4.5×10⁻⁴ to 6.5×10⁻⁴ mol/cm³ and especially preferably4.8×10⁻⁴ to 6×10⁻⁴ mol/cm³. When the network chain concentration iswithin this range, a glove is not easily broken, can easily be donnedand put off, and has good followability and good lastingness of thetight conformity to the skin of a wearer's hand.

[0058] The network chain concentration is determined by a method,explained in detail below, wherein a dip-formed article is dipped intoluene and, when the equilibrium swelling is reached, volume of theswollen specimen is measured, and the network chain concentration iscalculated from the volume of the swollen specimen and the originalvolume of the specimen as measured before the dipping in toluene.

[0059] The dip-formed article of the present invention has a thicknessin the range of about 0.05 mm to about 3 mm, and is especially suitablefor thin-gage filmy articles having a thickness of about 0.1 mm to about0.3 mm. As specific examples of the dip-formed article, there can bementioned a nipple of nursing bottle, medical articles such as adropper, a tube and a water-pillow: toys and sporting goods such as aballoon, a doll and a ball; industrial articles such as a pressure bagand a gas storage bag; a surgical glove, a household glove, anagricultural glove, a fishery glove and an industrial glove; and afinger cot. The dip-formed article is especially suitable for thin-gagesurgical gloves and thin-gage gloves for electronic parts-production.

[0060] The invention will now be described specifically by the followingworking examples that by no means limit the scope of the invention. Inthe working examples, % and parts are by weight unless otherwisespecified.

[0061] The physical properties evaluated in the working examples aredetermined by the following methods.

[0062] [Analysis of Composition of Copolymer]

[0063] A sample was taken from an as-produced copolymer latex, andresidual unreacted monomers were removed therefrom. Then the sample wascoagulated and then dried according to JIS K 6392 to give a solidrubber. A bound acrylonitrile content was measured on the solid rubbersample.

[0064] About 0.2 g of the above-mentioned solid rubber sample wasprecisely weighed and dissolved in 100 ml of pyridine. The rubber samplesolution was titrated with an 0.02N alcoholic potassium hydroxidesolution in a nitrogen atmosphere by using a thymolphthalein indicatorto neutralize the carboxyl groups in the copolymer. Thus, boundmethacrylic acid content was determined from the amount of potassiumhydroxide required for neutralization of the carboxyl groups in thecopolymer.

[0065] Bound 1,3-butadiene content in the solid rubber was determined asthe rest of sample, obtained by subtracting the sum of boundacrylonitrile content and bound methacrylic acid content from the weightof sample.

[0066] [pH Value]

[0067] pH value of a copolymer latex was measured at a temperature of25° C. and a solid concentration of 45% by using a pH meter (M12,available from HORIBA Co.).

[0068] [Weight Average Molecular Weight]

[0069] A copolymer latex having a solid concentration of 45% and pH of8.3 was prepared. The copolymer latex was cast on a glass sheet with aframe, and left to stand at a temperature of 23° C. and a relativehumidity of 50% for 48 hours to give a dry film. The dry film wasdissolved in tetrahydrofuran (THF). Using the solution, weight averagemolecular weight of the copolymer was determined by gel permeationchromatography (eluting solution: THF) using a calibration curveprepared on the basis of standard polystyrene.

[0070] [Insolubles in Methyl Ethyl Ketone]

[0071] 0.3 g of the above-mentioned dry film was placed in a cage ofwire mesh with 80 mesh, and dipped in 100 ml of methyl ethyl ketone at atemperature of 20° C. for 48 hours. The solid remaining undissolved inthe cage was dried under a reduced pressure at a temperature of 100° C.The dried solid was weighed to determine the content of insolubles inmethyl ethyl ketone.

[0072] [Network Chain Concentration]

[0073] Network chain concentration of a dip-formed article wasdetermined by the method described in Rubber Testing Method, NewEdition, Corporated Body “The Society of Rubber Industry, Japan”published Nov. 1, 1980, p211. This method will be described in detailbelow.

[0074] A circular test sample having a diameter of 2 cm (D1) was punchedfrom a dip-formed article having a glove shape with a thickness of 0.1to 0.13 mm. The test sample was thoroughly dipped in a large amount oftoluene, and left to stand at a temperature of 20° C. for 72 hours.After the dipped sample came to the equilibrium swelling, the diameter(D2) of the swollen sample was measured. Volume fraction (Vr) of rubberingredient in the swollen sample was calculated from the followingformula (1):

Vr=(D1/D2)³  (1)

[0075] Then the swollen sample was dried, and volume (g2) of the driedsample was measured. Volume fraction (g) of rubber ingredient in thesample before dipping in toluene was calculated from the ratio (g2/g1)wherein g1 is volume of the sample as measured before immersion intoluene.

[0076] Network chain concentration (τ) of the sample before dipping intoluene was calculated from the following formula (2):

τ=−g[1n(1−Vr)+Vr+μVr ² ]/V(g ^(⅔) Vr ^(⅓) −Vr/2)  (2)

[0077] where V: molar volume of toluene

[0078] μ: solvent-polymer interaction constant=106.24 cm³/mol

[0079] The solvent-polymer interaction constant μ was calculated fromthe following formula (3) according to Polymer Handbook Fourth Edition,VII/679.

μ=V([Sp] ₁ −[Sp] ₂)² /RT  (3)

[0080] where [Sp]₁: solubility parameter of toluene=9.1 (cal/cm³)^(½)

[0081] [Sp]₂: solubility parameter of rubber ingredient [(cal/cm³)^(½)]

[0082] R: gas constant [(cal/K·mol)]

[0083] T: absolute temperature (K)

[0084] The solubility parameter ([SP]₂) of rubber ingredient wascalculated from the following formulae (4) and (5) according to theFedors' method (Robert F. Fedors, Polym. Eng. Sci., 14 [2] 147 (1974).

Spi=(ΣΔej/ΣΔvj)^(½)  (4)

[Sp] ₂ =ΣXi×Spi  (5)

[0085] where Spi: solubility parameter of monomer unit “i”[(cal/cm³)^(½)]

[0086] Δej; evaporation energy (cal/mol) of atom (j) or atomic group (j)constituting monomer unit “i”

[0087] Δvj: mol volume (cm³/mol) of atom (j) or atomic group (j)constituting monomer unit “i”

[0088] Xi: Mol fraction of monomer unit “i” in rubber ingredient

[0089] [Preparation of Test Sample for Evaluation of Physical Propertiesof Dip-Formed Article]

[0090] A test sample with a thickness of 0.1 to 0.13 mm was punched froma glove-shaped dip-formed article by using a Die-c to a dumbbell shapeaccording to ASTM-D412.

[0091] [Modulus in Tension at 300% Elongation; 300% Modulus]

[0092] Modus in tension at 300% elongation was measured by usingtensilometer (“RTC-1225” available from Orientec K. K.) at a drawingrate of 500 mm/min.

[0093] [Tensile Strength]

[0094] Tensile strength at break was measured by using a tensilometerimmediately before a test sample was broken when it was drawn at adrawing rate of 500 mm/min.

[0095] [Elongation at Break]

[0096] Elongation at break was measured by using a tensilometerimmediately before a test sample was broken when it was drawn at adrawing rate of 500 mm/min.

[0097] [Modulus Retention]

[0098] Modulus in tension was measured immediately after a test samplewas drawn to an elongation of 100% and after the drawn sample wasretained for 6 minutes. After the specimen was retained for 6 minutes,its modulus in tension (B) was measured. The modulus retention wasexpressed by the ratio (B)/(A) of the modulus in tention (B) as measuredafter 6 minutes' retention to the modulus in tention (A) as measured at100% elongation.

[0099] [Physical Properties of Glove]

[0100] Ease in donning and putting off (ease in donning), weariness of awearer's hand when a glove is worn for a long time (i.e.,followability), and lastingness of tight conformity to the skin of awearer's hand (i.e., lastingness of tight conformity) were evaluated ondip-formed gloves according to a sensory analysis by ten wearers. Theresults were expressed by the average value of evaluation points of theten wearers determined according to a five point rating method. Thelarger the average value, the better the particular property. Breakingof glove was expressed by the number of wearers among ten wearers, whobroke a glove upon donning.

PRODUCTION EXAMPLE 1

[0101] A polymerization vessel, flushed with nitrogen, was charged with22 parts of acrylonitrile, 75 parts of 1,3-butadiene, 3 parts ofmethacrylic acid, 0.6 part of t-dodecyl mercaptan (TDM), 115.5 parts ofsoft water, 3.0 parts of sodium dodecylbenzenesulfonate, 0.5 part of asodium salt of β-naphthalenesulfonic acid-formaldehyde condensate, 0.3part of potassium persulfate and 0.05 part of ethylonediaminetetraaceticacid, and the content was maintained at 37° C. to initiatepolymerization. After the reaction was carried out for 20 hours, areaction stopper was added to complete the polymerization. At this time,the polymerization conversion was 92%. A portion of the thus-obtainedcopolymer latex was taken to analyze the composition of copolymer. Theresults of analysis are shown in Table 1.

[0102] Unreacted monomers were removed from the copolymer latex, and pHand solid content of the copolymer latex were adjusted to give acopolymer latex “A” having a solid concentration of 45% and a pH valueof 8.3.

PRODUCTION EXAMPLES 2 to 6

[0103] The procedures described in Production Example 1 were repeatedwherein the composition of monomers was varied as shown in Table 1 togive copolymer latexes “B” through “F”, all of which had a solidconcentration of 45% and a pH value of 8.3. TABLE 1 Production Example 12 3 4 5 6 Monomers (parts) Acrylonitrile 22 27 32 42 27 22 1,3-Butadiene75 71 66 54 67.5 77.9 Methacrylic acid 3 2 2 4 5.5 0.1 Copolymercomposition (%) Acrylonitrile 23 27.5 33 41 28 23 1,3-Butadiene 74 70 6535 66.5 76.9 Methacrylic acid 3 2.5 2 4 5.5 0.1 Copolymer latex A B C DE F Weight average molecular weight (×1000) 143 123 139 205 141 137Insolubles in MEK (%) 3 2.5 1.3 2.2 1.9 2

EXAMPLE 1

[0104] 3 parts of sulfur, 1.5 parts of titanium oxide, 1.5 parts of zincdiethyldithiocarbamate, 1.5 parts of zinc 2-mercaptpbenzothiazole, 0.3part of a 45% solution of sodium salt of β-naphthalenesulfonicacid-formaldehyde condensate, 0.002 part of potassium hydroxide and7.472 parts of water were mixed together by using a ball mill to preparea vulcanizer dispersion having a solid content of 50%. Solid content andpH of copolymer latex A prepared in Production Example 1 were adjustedto prepare a copolymer latex having a solid concentration of 30% and apH value of 10. 333.3 parts of this copolymer latex was mixed with 15parts of the above-mentioned vulcanizer dispersion to prepare adip-formable composition.

[0105] 25 parts of calcium nitrate, 5 parts of calcium carbonate, 0.025part of polyoxyethylene octyl phenyl ether and 70 parts of water weremixed together to prepare a coagulating solution having solid content of30%. A glove form was dipped in the coagulating solution, taken outtherefrom and dried to deposit a coagulant on the glove form.

[0106] The coagulant-deposited glove form was dipped in theabove-mentioned dip-formable composition, and then taken out therefrom.The thus-obtained glove form coated with the dip-formable compositionwas dried by a dryer. Then the temperature was elevated to 120° C. andthe dried glove form was heat-treated at that temperature for 20 minutesto give a glove form having a solid rubber film on the outer periphery.Then the solid rubber film was stripped from the glove form to give adip-formed article with glove shape. The physical properties of theglove were evaluated. The results are shown in Table 2. TABLE 2Comparative Example Example 1 2 3 1 2 3 Dip-formable composition (parts)Copolymer latex A B C D E F Amount of latex 100 100 100 100 100 100 Amt.of sulfur 3 3 3 3 3 3 Ant. of zinc oxide 0 0 0 0 0 0 Net-work chainconcentration (×10⁻⁴ mol/cm³) 5.1 4.5 5.3 9.6 7.3 2.9 Properties ofdip-formed articles 300% Modulus (MBa) 1.9 1.7 2.2 9.1 3.7 0.71 Tensilestrength (MPa) 17.5 17.8 18.5 37.9 27.6 1.5 Breaking elongation (%) 620650 620 470 500 520 Modulus retention (%) 63 66 70 42 46 77 Propertiesof glove (points) Non-weariness of hand 4.6 4.4 4 1 1.8 5 Breaking ofglove 0 0 0 0 0 10 Ease in donning 4.4 4.9 4.5 2.7 3.5 3.5 Tightconformity 4.2 4.4 4.6 2.6 2.8 5

EXAMPLES 2 TO 5, COMPARATIVE EXAMPLE 1 TO 6

[0107] By the same procedures as described in Example 1, dip-formedgloves were made wherein the kind of copolymer latex, and amounts ofsulfur and zinc acetate were varied as shown in Table 2 and Table 3.TABLE 3 Comparative Example Examples 4 5 4 5 6 Dip-formable composition(parts) Copolymer latex A A A A A Amount of latex 100 100 100 100 100Amt. of sulfur 2.5 4 7 1 4 Amt. of zinc oxide 0 0.1 0 0 0.3 Networkchain concentration (×10⁻⁴ mol/cm³) 5.2 6.4 8.2 3.5 7.8 Properties ofdip-formed article 300% Modulus (MPa) 1.8 2.4 5.6 1.2 3.2 Tensilestrength (MPa) 14.1 15.2 16.1 6.5 8.9 Breaking elongation (%) 600 550400 650 420 Modulus retention (%) 60 68 81 45 71 Properties of glove(points) Non-weariness of hand 4.4 3.9 1.6 4.8 2.5 Breaking of glove 0 00 4 2 Ease in donning 4.2 4 2 5 2.3 Tight conformity 4 4.5 5 2.8 4.6

[0108] From comparison of gloves of the present invention with those ofcomparative examples, shown in Table 2 and Table 3, it will be seen thatthe gloves of comparative examples are characterized as follows.

[0109] The glove, made in Comparative Example 1, having large 300%modulus, small breaking elongation and small modulus retention has poorease in donning, poor non-weariness of hand, and poor lastingness oftight conformity.

[0110] The glove, made in Comparative Example 2, having large 300%modulus and small modulus retention has poor ease in donning, poornon-weariness of hand, and poor lastingness of tight conformity.

[0111] The glove, made in Comparative Example 3, having extremely lowtensile strength is easily broken upon donning.

[0112] The glove, made in Comparative Example 4, having large 300%modulus and small breaking elongation has poor ease in donning and poornon-weariness of hand.

[0113] The glove, made in Comparative Example 5, having low tensilestrength and small modulus retention is easily broken upon donning andpoor lastingness of tight conformity.

[0114] The glove, made in Comparative Example 6, having large 300%modulus, low tensile strength and small breaking elongation is easilybroken upon donning and has poor ease in donning and poor non-wearinessof hand.

[0115] In contrast, the gloves, made in Examples 1 to 5, having thecharacteristics satisfying the requirements of the present invention arenot easily broken upon donning, and has enhanced ease in donning, goodnon-weariness of hand, and good lastingness of tight conformity.

[0116] Industrial Applicability

[0117] The dip-formed article of the present invention, made from adip-formable composition comprising the specific conjugated dienecopolymer and sulfur, and having the specified properties, ischaracterized as a glove by being well elongated and ease in donning,being not easily broken upon donning, and not wearying a wearer's handeven when it is worn for a long time, and having improved lastingness ofthe tight conformity to the skin of a wearer's hand.

1. A dip-formed article which is shaped from a dip-formable compositioncomprising a conjugated diene copolymer prepared by copolymerization ofa conjugated diene monomer with an ethylenically unsaturated nitrilemonomer and an ethylenically unsaturated acid monomer, and sulfur; andwhich has a modulus in tension at 300% elongation of not larger than 3MPa, a tensile strength at break of at least 10 MPa, an elongation atbreak of at least 500% and a modulus retention of at least 50%.
 2. Thedip-formed article according to claim 1, which has a network chainconcentration in the range of 4×10⁻⁴ to 7×10⁻⁴ mol/cm³.
 3. Thedip-formed article according to claim 1, wherein said conjugated dienecopolymer comprises 50% to 85% by weight of conjugated diene monomerunits, 15% to 40% by weight of ethylenically unsaturated nitrile monomerunits and 1% to 4% by weight of ethylenically unsaturated acid monomerunits, and said network chain concentration of the dip-formed article isin the range of 4.8×10⁻⁴ to 6×10⁻⁴ mol/cm³.
 4. The dip-formed articleaccording to claim 1, which is a medical article, a toy, a sports good,an industrial article or a glove.
 5. The dip-formed article according toclaim 1, which is a thin-wall glove.
 6. A dip-formable compositioncomprising 100 parts by weight of a conjugated diene copolymercomprising 50% to 85% by weight of conjugated diene monomer units, 15%to 40% by weight of ethylenically unsaturated nitrile monomer units, 1%to 4% by weight of ethylenically unsaturated acid monomer units and 0%to 30% by weight of copolymerizable other ethylenically unsaturatedmonomer units, 2 to 6 parts by weight of sulfur and 0 to 0.2 part byweight of zinc oxide.
 7. The dip-formable composition according to claim6, which further comprises 0.5 to 4 parts by weight of a vulcanizationaccelerator, based on 100 parts by weight of the conjugated dienecopolymer.
 8. The dip-formable composition according to claim 6, whereinsaid conjugated diene copolymer is prepared by emulsion polymerizationand contains smaller than 50% by weight of ingredients insoluble inmethyl ethyl ketone.
 9. The dip-formable composition according to claim6, wherein said conjugated diene copolymer has a weight averagemolecular weight in the range of 50,000 to 500,000.
 10. A method formaking a dip-formed article comprising dip-forming a dip-formablecomposition as claimed in claim 6.